I've never thought about the medium thing. If you write a song (and perhaps perform it in public and everything), but never commit it into writing or recording, does copyright not apply?
If you wrote it on paper, whether musical notation or just the lyrics, that would give you songwriter copyright.
If you then gave a performance and someone recorded it, the copyright in the recording would lie with them - but they would not be able to distribute it without also having your permission.
(People forget that these are different and the songwriter royalties are quite lucrative - famously e.g. the Beatles almost all the songs are joint copyright Lennon/McCartney, not the band as a whole)
Yes, sure, if you record a riff or write a few sentences on paper, then you own the copyright to that - there is no "magic importance limit" that needs to be surpassed for works to become copyrightable.
Shitty drawings made by five year olds get the same legal treatment as Picasso, etc, etc.
If you publish it. If you just sit on it, you don't. (At least not in the sense that you can get away with suing someone that publishes the same rift later)
IANAL, but unlike patents, copyright violation only occurs if you actually copy the work - independent creations are not infringing, even if similar. While the standard for showing that the work was actually copied is usually not high, especially since these are usually civil suits, some link in a random Google Drive might not be enough.
I'd guess that standard would be set by a court case, unless there already has been one like this (in which case please enlighten me). In general, with vagueness like this, a court usually has the final say.
There's no such thing, copyright is automatic, it applies as soon as the author puts the work in some tangible medium (like an hard drive).